This invention relates in general to a process for recovering energy from liquefied gases by evaporation of same in heat exchange with a cycle medium which is simultaneously cooled thereby.
Processes for recovering energy from liquefied natural gas (LNG) are well known. In one of these processes (U.S. Pat. No. 2,937,504, for example), LNG is evaporated in heat exchange with propane serving as the cycle medium, and during this procedure, the propane is liquefied. In another conventional process (U.S. Pat. No. 3,068,659), ethane is used as the cycle medium, which is likewise liquefied by heat exchange with the liquefied natural gas.
In these known processes, a common feature is that the cycle medium, after liquefaction by heat exchange with liquefied natural gas, is compressed by a pump, then expanded and revaporized by being heated with an external source of heat. In such systems, high pressures are necessary to recover the maximum amount of liquefaction energy from the natural gas; in the high-pressure section of the cycle system, these pressures can be, for example, up to 140 bars (U.S. Pat. No. 3,068,659). The use of such high pressures, however, means that the temperature level on the warm side of the cycle system -- similarly as in a steam power plant -- remains limited and thus the supplied external heat is not used in an optimum manner. Primarily for this reason, none of these prior art processes attains the degree of efficiency of a well designed and operated conventional power plant.
Another substantial disadvantage of these known processes is that they cannot ordinarily stay on stream if the supply of natural gas is interrupted, since the cycle medium cannot be liquefied in a simple manner at ambient temperature.
Still another disadvantage of the conventional method is that the cycle media are all combustible substances, thereby requiring, especially in the high-temperature range, stringent controls and precautions to reduce the explosion hazards to the minimum extent possible.